Reduce and optimize Group rebuilds

[Benno]

🆕 Feature Request

❓ Is your feature request related to a problem?

If you have a very very big Group and you mutate its value.. it rebuilds the whole Group output on a single change which might result in performance leaks.

📄 Describe the solution you'd like

Rebuild the Group only if the built output is needed.. for instance when calling output, or items

📃 Describe alternatives you've considered

bad performance

[Benno]

Tried to optimize the group rebuilds in merge request #189.
But I was not yet able to make the group noticeably faster :/